We propose to use a nonhuman primate separation paradigm as an animal model system investigate the relationship of social support, circadian rhythm regulation, drug treatment and phototherapy for prediction/prevention and treatment of the adverse psychiatric and medical consequences of separation and loss experiences. We will specifically test the following hypotheses: 1) the presence of social support will ameliorate the adverse physiological, immunological, and behavioral responses to separation and loss, whereas the absence of social support will accentuate the adverse response: 2) the adverse physiological and immunological consequences of social separation experiences can be prevented by treatment with tricyclic drugs; 3) measurement of individual differences in internal desynchrony of X and Y circadian rhythms forced by a sudden phase advance of the light/dark zeitgeber, as well as the response to a free-running period of 60 hours free of zeitgebers, will predict individual differences in physiological, immunological and behavioral responses to social separation; and 4) early morning bright light treatment (phototherapy) will prevent the biological rhythm changes and physiological and immunological consequences of separation experiences. All studies will utilize mother-reared social group living Macaca nemestrina and Macaca radiata monkey infants undergoing brief maternal separations. Physiological data will be obtained by means of multichannel implantable biotelemetry systems. Immunological data will be obtained from biweekly venipuncture. Behavioral data will obtained by observers coding behavior on a computer terminal in real time. Our findings will improve our understanding of how social support acts as a buffer the stress of social separation in the precipitation of depressive symptoms, and they will be relevant to how social support may buffer the adverse physiological and immunological consequences of separation experiences in HIV- antibody positive patients prior to conversion to AIDS or ARC. The findings will also determine the extent to which individual differences in the control of circadian rhythms predict physiological and immunological response to loss, and whether appropriately timed phototherapy can act as an effective therapeutic intervention to prevent the adverse physiological and immunological consequences of separation and loss.